1. Field of the Invention
The present invention relates to a load drive device for driving constant load devices, such as LEDs (Light Emitting Diodes), electric lamps, motors or the like that have little or no abrupt load variations, particularly to a load drive device suitable for use in a power control circuit aimed at expanding a power control range.
2. Description of the Related Art
There has been disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2009-33090, a load drive device comprising a converter performing PWM (Pulse Width Modulation) control according to a power control signal input to thereby adjust powers of loads. As an example of such a load drive device, FIG. 8 shows a conventional example of a light-emitting element drive device whose load comprises a plurality of series-connected LEDs 10.
In FIG. 8, numerical symbol 12 denotes a DC power supply, and numerical symbol 14 denotes a converter circuit for boosting an input voltage Vin from the DC power supply 12 to a DC output voltage Vout. An output terminal of the converter circuit 14 is connected to a series circuit of the LEDs 10 and a shunt resistor 16 serving as a current detector. The converter circuit 14 is composed of a choke coil 20, a main switching element 22 such as a MOSFET or the like, a diode 24 and a capacitor 26. The converter circuit 14 allows the input voltage Vin to be applied to the choke coil 20 when the main switching element 22 is turned on, thereby allowing an electric energy to be stored in the choke coil 20. When the main switching element 22 has been turned off, the electric energy stored so far in the choke coil 20 and an electric energy from the DC power supply 12 are sent out to the capacitor 26 on an output side, through the diode 24, thus allowing the output voltage Vout higher than the input voltage Vin to be supplied to the LEDs 10.
Meanwhile, a feedback circuit 28 for performing constant current control of the LEDs 10, comprises: an error amplifier 32 for comparing a voltage of a current detection signal of the LEDs 10 detected through the shunt resistor 16 with a reference voltage from a reference voltage source 30 and performing error amplification; a phase compensation circuit 38 composed of a series circuit of a resistor 34 and a capacitor 36 that are connected between an output terminal of the error amplifier 32 and a ground line to prevent the oscillation of the error amplifier 32, and; an oscillation circuit 40 for generating a reference signal of a triangle waveform signal or a saw-tooth waveform signal; and a comparator 42 for supplying to a gate of the main switching element 22 the result of comparison of an error amplification signal from the error amplifier 32 with the reference signal from the oscillation circuit 40, as a pulse drive signal. Among these components, the oscillation circuit 40 and the comparator 42 make up a PWM (Pulse Width Modulation) circuit 44 for generating the pulse drive signal with a duty ratio corresponding to the voltage level of the error amplification signal.
Under a steady-state condition, a current flowing through the LEDs 10 is detected by means of the shunt resistor 16, and a current detected value from the shunt resistor 16 is then compared with the reference voltage by the error amplifier 32, thus performing error amplification by the error amplifier 32. The error amplification signal thus generated is allowed to pass through the phase compensation circuit 38, and the pulse drive signal is generated based on the error amplification signal passing through the phase compensation circuit 38 in the PWM circuit 44, thereby allowing the main switching element 22 to be driven by the pulse drive signal, thus controlling the current flowing through the LEDs 10 to an objective output (current) value.
Numerical symbol 46 denotes a burst signal generation circuit for generating a burst signal based on a dimming signal supplied from external, said burst signal having a square waveform with a frequency lower than that of the pulse drive signal. The duty ratio of the burst signal is determined in accordance with an analog voltage level of the dimming signal, and the higher the voltage level of the dimming signal is, the smaller the duty ratio of the burst signal generated. Further, the dimming signal is output from the burst signal generation circuit 46 as it is, when the pulse-width modulated dimming signal other than a DC voltage, is input.
The PWM circuit 44 is allowed to output the pulse drive signal only when the burst signal is active. In other words, the PWM circuit 44 is configured so as not to output the pulse drive signal when the burst signal is inactive. For example, outputting the pulse drive signal stops by dropping a voltage input to an inverting input terminal of the comparator 42 or by stopping operation of the PWM circuit 44. Further, a switch element 48 is connected between the output terminal of the error amplifier 32 and the phase compensation circuit 38. The switch element 48 is turned on while the burst signal is active, but turned off while the burst signal is inactive. In this case, when the switch element 48 is turned off, the charges stored in the capacitor 36 of the phase compensation circuit 38 are held therein, thereby retaining the previous value of the error amplification value (a control value) caused by the previous operation of the burst signal, thus expanding the dimming range of the LEDs 10.
Conventionally, as the burst signal for PWM dimming becomes active, the switch element 48 is turned on, thereby ceasing to hold the previous value held by the capacitor 36 of the phase compensation circuit 38, and a controlling value Co input to the comparator 42 starts with the previous value.
FIG. 9 is a timing chart showing a correlation between an output current Iout detected through the shunt resistor 16 and the controlling value Co input to the comparator 42. Here, there are shown waveforms of the output current lout and the controlling value Co in a case where the duty ratio of the burst signal is 40% (an upper row in FIG. 9), and in a case where the duty ratio of the burst signal is 5% (lower row in FIG. 9). As the duty ratio of the burst signal becomes small, a active period of the burst signal becomes short. Therefore, before the output current Iout reaches a objective value, the burst signal becomes inactive due to a delay of the phase compensation circuit 38 and delays of the choke coil 20 and the capacitor 26 that compose the converter circuit 14. In this way, the output current lout at that time becomes lower than that output under steady state condition, thereby degrading the linearity of variable output power when decreasing a luminance of the LEDs 10 through dimming control. Therefore, conventionally, a lower limit of the output power supplied to the LEDs 10 has to be so limited that, for example, the duty ratio of the burst signal should be at least 10%.